iv PRONEPHROS 231 



This posterior nephrocoele is still in wide communication with tin* 

 splanchnocoele indirectly by way of the anterior nephrocoele. 



DIPNOI. In Lepidosiren and Protopterus 1 the fully functional 

 pronephros of the larva possesses usually two tubules (Fig. 128, B and 

 D). These are the surviving members of a series of tubule rudiments 

 extending through at least the anterior 4-7 segments but probably 

 extending much further back. The tubules which In-come fully 

 developed are normally "B" and "D" i.e. those corresponding to 

 the second and fourth mesoderm segments. Thus the second tubule 

 does not correspond with the second tubule in the fully developed 

 pronephros of Polypterus. The tubules appear to originate (cf. Fig. 

 129, A, B) as in Hypogeophis except that the outgrowths from the 

 nephrotomes are solid as in Polypterus and such is the case also 

 with the archinephric duct rudiment. 



The nephrocoeles of the two main pronephric tubules undergo 

 fusion as in Polypterus so as to form a large pronephric chamber on 

 each side. This is continuous with the pericardiac portion of the 

 splanchnocoele and the two glomeruli as usual become fused together to 

 form a compound glomerulus. 2 In Lepidosiren the fusion of the pro- 

 nephric chambers takes place before the appearance of the glomerular 

 rudiments. These appear first on the floor of the continuous cavity 

 (Fig. 129, C, gl) and very soon undergo fusion themselves. By 

 differential growth the root of attachment of the glomerulus becomes 

 gradually shifted towards the mesial plane and dorsally (Fig. 129, D 

 and E) so that it comes to hang down into the pronephric chamber 

 or nephrocoele from a point in close proximity to the dorsal 

 aorta. 



The pronephric chambers are at first perfectly continuous with 

 the splanchnocoele which spreads outwards from them. Later on the 

 pronephros becomes greatly enlarged and bulges across the splanchno- 

 coele until it comes in contact with the mesodermal sheath of the 

 oesophagus. Fusion then takes place (at the point marked with * * in 

 Fig. 129, E) between the surfaces in contact so that the glomerulus 

 conies to be enclosed in a secondary pronephric chamber, which 

 however remains freely open to the splanchnocoele at its hinder end. 

 The glomerulus becomes firmly slung diagonally across this chamber 

 by its tip undergoing fusion on the ventrolateral side of the chamber 

 with the mesoderm investing the pronephros. 



In Ceratodus (Semou, 1901) the pronephros probably develops in 

 a manner similar to that described in the case of the other two Lung- 

 fishes. The organ in its first stage is a solid projection of the meso- 

 derm the appearance in section being similar to that figured for 

 Lepidosiren. The portions of the rudiment corresponding to the 

 individual tubules are in such close apposition as to be at first 

 indistinguishable (as is often the case in the other two Lung-fishes) : 



1 A large part of the investigations upon which this account is based were carried 

 out by Miss Muriel Robertson in the University of Glasgow during 1904. 

 ' J The word glomus is often used for such a compound glomerulus. 



